Flash in Phones and Tablets: One Board, Two Memories

In February 2015, Samsung began mass producing ePoPAn ePoP is a single memory package that holds mobile DRAM, eMMC flash and a controller, stacked directly on the mobile processor.. It was a single package of 3GB of LPDDR3 DRAM, 32GB of eMMC and a controller, and it sat directly on top of a phone’s processor. Two memories that had needed two footprints on the board now needed one. How a maker packages them decides what a spec sheet shows and what a buyer can choose.

Part 1, UFS Explained, covers the link between a phone and its flash, and Part 2, eMMC Explained, covers the older one. This part covers what surrounds both of them, meaning how flash and LPDDRLPDDR is the low-power branch of DDR memory for phones and laptops. It is soldered close to the processor and built to idle at very little power. share a package, how makers pair storage with RAM, and what Android does to keep working data out of flash and stored data encrypted.

Two memories share one board

LPDDR is DRAM designed for low power, and a phone keeps its working data there. Whatever has to survive a power cut goes in flash. Micron’s 1-gamma LPDDR5X is sampling with a 10.7Gb/s speed grade, against 7.5Gb/s for its earlier 1-beta LPDDR5X, in what Micron calls the industry’s thinnest LPDDR5X package. Samsung’s newly developed 10.7Gb/s LPDDR5X allows a single mobile DRAM package of up to 32GB. Flash sits behind its own interface, which Part 1 covers.

Figure 1 shows the conventional arrangement. In a package-on-package (PoPA package-on-package stack puts a DRAM package on top of the processor package. In the conventional layout the flash is a separate package beside the stack.) stack, the DRAM package sits on top of the processor package, and the flash is a separate package beside them. Foundry-made versions of the stack are in Photolithography Part 6. Samsung’s description of a conventional PoP puts the processor and DRAM stack at 15 × 15mm and the eMMC beside it at 11.5 × 13mm.

ePoP and uMCP put both memories in one package

Samsung’s ePoP replaces that arrangement with one package. It holds 3GB of LPDDR3 running at 1,866Mb/s over a 64-bit interface, 32GB of eMMC and a controller. It takes no board space beyond the 225mm² the processor already occupies, which cuts the area of the conventional layout by about 40%. The height limit of 1.4mm is met as well. Samsung added that it already sold a similar package for wearables, which it called wearable memory, and that the phone version could serve high-end tablets.

Micron took a different route. On October 20, 2020, it launched uMCP5A uMCP is a multichip package that pairs UFS flash storage with LPDDR DRAM in one package., which it called the first multichip package to pair UFS storage with LPDDR5. It uses UFS 3.1, and a phone built around it saves 55% of circuit board space against separate LPDDR5 and UFS parts, by Micron’s count. Its DRAM bandwidth reaches up to 6,400Mb/s, 50% above the 4,266Mb/s of the previous LPDDR4x generation.

Figure 2 sets the three layouts side by side. The two percentages have different baselines. Samsung’s 40% is against a PoP stack plus a separate eMMC, and Micron’s 55% is against stand-alone LPDDR5 and UFS parts, so they do not compare with each other. uMCP5 comes in four pairs. They are 128+8GB, 128+12GB, 256+8GB and 256+12GB, with NAND first and LPDDR5 second. Its NAND is rated for 5,000 program/erase cycles, an improvement of around 66% in Micron’s words.

A spec sheet that lists 12GB and 256GB does not say which layout holds them, and that is the maker’s documentation to read.

Storage tiers come with fixed amounts of RAM

Micron sold its package in fixed pairs, and tablets show the same habit from the buyer’s side. Apple’s specifications for the iPad Pro 13-inch (M4) show 8GB of RAM on the 256GB and 512GB models and 16GB on the 1TB and 2TB models. Memory bandwidth is 120GB/s on every tier, so the larger storage tiers get twice the RAM and the same bandwidth.

Figure 3 plots Apple’s pairs. A buyer who wants the larger RAM has to choose a larger storage tier first. Storage also gates a feature, since the same page allows ProRes video recording up to 4K at 30fps and limits the 256GB model to 1080p at 30fps. Apple’s page gives no reason for either pairing. A footnote defines 1GB as 1 billion bytes and warns that the available space is less than the listed capacity.

Android keeps working data out of flash

Android’s developer documentation names three kinds of memory on a device, RAM, zRAMzRAM is a part of RAM that serves as compressed swap space. The kernel can move cached pages into it, where they are compressed, which frees RAM. and storage. A part of RAM, zRAM, serves as compressed swap space, and Android does not use storage for swap because frequent writing wears the flash and shortens its life. Memory is managed in pages of typically 4KB. When RAM runs short, the kernel’s kswapd can move cached private dirty pages and anonymous dirty pages into zRAM, where they are compressed, and a page is not written out to flash to make room.

Figure 4 shows where the line falls. When compression is not enough, Android kills processes. Early versions relied on a driver inside the kernel for that, and since kernel 4.12 the low memory killer daemon, lmkd, has done it from user space. This design protects the flash by spending RAM.

F2FS writes the way flash prefers

Android’s own examples of file-based encryptionWith file-based encryption, each Android user has Credential Encrypted storage, available only after unlock, and Device Encrypted storage, which is available during Direct Boot as well. mount the user data partition as F2FSF2FS is a log-structured file system built for NAND flash storage such as SSDs, eMMC and SD cards.. The kernel documentation describes the Flash-Friendly File System as built for NAND flash storage such as SSDs, eMMC and SD cards, and it is log-structured. A log-structured file system writes all modifications sequentially into a log, divides the log into segments, and uses a cleaner to gather the live data out of fragmented segments.

Figure 5 shows the six logs F2FS keeps open at once, hot, warm and cold for its node blocks and for its data blocks. Directory entries go to the hot data log, and the cold data log takes multimedia and migrated blocks. Separating blocks by how often they change is one of the ways F2FS cuts cleaning overhead.

F2FS also lines itself up with the flash below it. It expects its section size to match the garbage-collection unit of the flash translation layer, and it spreads the sections of its active logs across zones where it can. Cleaning runs on demand when free segments run short, and in the background when the system is idle. With real-time discard enabled, F2FS sends discard (TRIM) commands for the segments it cleans. Part 1 covers the device’s own controller, which does the matching work on its side.

Encryption sits in the path to the flash

Android has supported file-based encryption (FBE) since version 7.0, and every device launching with Android 10 or higher must use it. Each user gets Credential Encrypted storage, which opens only after unlock, and Device Encrypted storage, which is also available during Direct Boot before the user unlocks. FBE runs on both the Ext4 and F2FS file systems.

Figure 6 follows a write down to the flash. Android recommends inline encryption hardware, which encrypts and decrypts data on its way to and from the storage device, to improve performance and cut power use. Common kernels 4.14 and higher carry the framework, with an option for UFS. Android 11 added hardware-wrapped keys. Only dedicated hardware knows the raw key, and software works with it in wrapped form while the hardware loads a subkey into the inline crypto engine, usually in the UFS or eMMC host controller. The aim is protection against cold boot attacks and against leaks of system memory.

Reading a phone or tablet specification

A line such as 12GB and 256GB gives two numbers and leaves three questions open. One is whether the RAM is tied to the storage tier, as it is on the iPad Pro. Another is whether the two memories share a package, which the maker’s documentation shows, as with ePoP and uMCP5. Last comes the storage interface and its version, which Part 1 sorts out.

Figure 7 puts the three questions beside a spec line. Capacity is a fourth check, since the maker’s count and the space a user can fill differ by the footnote above. Anyone comparing two tablets should compare the storage tier, the RAM that comes with it and the UFS version, and treat packaging as a detail the datasheet supplies when it matters.

Part 6, Flash in Small Devices, leaves the pocket for the routers, cameras and watches that boot from flash chips with a handful of pins.